1. Field of the Invention
This invention relates to an information recording device and an information recording method for recording information on a write-once type optical information recording medium by means of a laser beam.
2. Related Background Art
Optical information recording/reproduction devices typically including optical disc recording/reproduction devices are adapted to converge a laser beam onto the recording layer of a recording medium so as to write information to and read information from the recording layer. If the wavelength of the laser beam is λ and the numerical aperture of the objective lens is NA, the diameter (Φ) of the spot of converged light on the recording medium is generally expressed by formula (1) below.Φ=λ/NA  (1)
Therefore, the shorter the wavelength of the laser beam and the larger the numerical aperture of the objective lens, the small the diameter of the spot of converged light on the recording medium so that information can be recorded optically at high density. The numerical aperture of the objective lens is 0.45 in the case of CD (Compact Disc (tradename)) players and 0.6 to 0.65 in the case of DVD (Digital Versatile Disc (tradename)) players. The numerical aperture of the objective lens of recently developed BD (Blu-ray Disc (tradename)) recording/reproduction devices that use a blue-violet semiconductor laser (wavelength: 405 nm) is improved to be as large as 0.85.
Generally, two different control methods are provided for the purpose of controlling the power of irradiating a laser beam when writing data They are selectively used depending on the type of the receiving material. A laser beam is irradiated continuously to form a recording mark with one of the control methods for controlling the power of irradiating a laser beam. This method is mainly applied to write-once type recording mediums formed by using an organic coloring material. With this method, a pulse beam is irradiated for a single mark (a part of EFM signal where the signal is at HIGH) of an EFM (Eight-to-Fourteen Modulation) signal, as shown in FIG. 15. A pulse beam is irradiated once or for a plurality of times to form a recording mark (pulse irradiation method) with the other control method for controlling the power of irradiating a laser beam. This method is mainly applied to rewritable type recording mediums formed by using a phase change recording material. For example, a pulse beam is irradiated for a plurality of times for a single mark (a part of EFM signal where the signal is at HIGH) of an FEM signal as shown in FIG. 16.
In the case of a recording/reproduction device for CD-RWs that are recording mediums formed by using a phase change recording material, a system of irradiating a pulse beam for n−1 times or n−2 times in order to form a recording mark with a length of n×T as write strategy, in other words, a system defining a width of T/2 for the pulse beam (pulse irradiation system), is adopted. Note that an EFM (Eight-to-Fourteen Modulation) code is used as modulation code for CD-RWs and, in the above description, n represents an integer not smaller than 3 and not greater than 11, while T represents the cycle period of the clock (channel clock) of the binary signal (EFM signal) to be recorded.
As shown in FIG. 16, a write strategy of synchronizing the timing of emission of a pulse beam with the channel clock is adopted in recording/reproduction devices for CD-RWs to simplify the configuration of the pulse generation circuit for driving the semiconductor laser. Additionally, a write strategy of controlling the power of emitting a laser beam by way of three levels of a recording peak level (Pw), an erasing level (Pe) and a cooling level (Pb) for write operations so as to make it possible to record signals by overwriting by means of a single beam.
Meanwhile, in recording/reproduction devices for CD-RWs, the channel clock for the basic recording rate (1×1) is as low as 4.32 MHz and that the time length corresponding to the channel clock cycle period of (1×T) is about 230 ns. If a write strategy of defining a width of T/2 for the pulse beam to be irradiated is adopted as described above, the time length of the pulse beam to be irradiated is 115 ns. Then, an excellent recording/reproduction performance can be realized because this time length is sufficiently long for the rising time (tr) and the falling time (tf) (both tr and tf being normally 2 to 3 ns) of the waveform of a laser beam emitted from a semiconductor laser.
However, with the high speed recording rate (24 times) that is specified by the high speed recording specifications for CD-RWs and has been put to use in recent years, the channel clock is as high as about 104 MHz and the time length corresponding to the channel clock cycle period of (24×T) is about 10 ns. If a write strategy of defining a width of T/2 for the pulse beam to be irradiated is adopted as described above, the time length of the pulse beam to be irradiated is as short as about 5 ns. Then, it is difficult to drive the semiconductor laser to emit a pulse beam for such a short time length. In view of this problem, according to the high speed recording specifications for CD-RWs that defines a high speed recording rate of 24 times as high as the ordinary recording rate, a system of irradiating a single pulse to form a recording mark with a length of (2×T), or irradiating a pulse beam n/2 times or (n−1)/2 times to form a recording mark with a length of n×T (2T pulse recording system) is adopted as write strategy. Such a recording method is disclosed, inter alia, in Japanese Patent Application Laid-Open Publication Nos. 2002-288837 and 2002-237051.
FIG. 17A to FIG. 17D illustrates the 2T pulse recording system in detail. With the 2T pulse recording system, recording marks are classified into three categories of 3T mark (shortest mark) as shown in FIG. 17B, even mark (e.g., 8×T) as shown in FIG. 17C and odd mark (e.g., 9T) as shown in FIG. 17D depending on the pulse width of the write binary signal (EFM signal) as shown in FIG. 17A, to each of which a certain degree of freedom is allowed in terms of the position of the irradiated pulse beam and the pulse width when forming recording marks.
With the 2T pulse recording system, for instance, all the radiated pulses are synchronized with the phase of the channel clock for even marks as shown in FIG. 17C but, as shown in FIG. 17D, the values of the position of irradiation, the duration of irradiation and the cooling time after the pulse irradiation of the tail end pulse for odd marks are differentiated from those of the tail end pulse for even marks. This arrangement is devised to change the length of the recorded mark under the condition of a same number of times of pulse irradiation. Additionally, as for the category of 3T mark, a single irradiated pulse and the cooling time can be freely defined so as to synchronize it with other recording marks on the recording medium in terms of recording mark length and edge position.
Efforts have been paid in recent years to develop write-once type recording mediums that are compatible with the BD (Blu-ray Disc (tradename)) system that employs a blue-violet semiconductor laser (e.g., wavelength: 405 nm). Write-once type recording mediums contain an organic coloring material or a metal material in the recording layer thereof so that they allow irreversible recording (write-once recording).
To keep pace with the development of write-once type recording mediums, efforts are also being paid to develop optimum write strategies to be used for write-once type recording mediums adapted to the BD system. When a pulse irradiation system is adopted for a write strategy for write-once type recording mediums adapted to the BD system, it may be safe to assume that it is necessary to keep uniform pulse irradiation intervals for the purpose of achieving an optimum recording/reproduction performance.
While each of the above cited patent documents describes a method of applying the 2T pulse recording system, the patent documents are limited to the use of reversible phase change recording materials for recording mediums and, according to them, pulse irradiation intervals are not held uniform so that they may not feasibly be able to find practical applications. While the International Patent Application Laid-Open Publication No. WO03023771 proposes a similar pulse recording method, the proposed method is adapted to control the mark length mainly by the irradiated tail end pulse width so that an excellent recording/reproduction performance may not be achieved if the method is applied to write-once type recording mediums.